When performing a lift in the hoisting and rigging industry, it is often desired that the load be in equilibrium so that each connection point at the load carry an equal part of the weight at all times to prevent overloading of the lifting components or prevent undesirable stresses on the load. In some applications, not only is it necessary to keep the load points in equilibrium, it is necessary to rotate or tilt the load from a horizontal position to a vertical position. Commonly, this task is accomplished by using wire rope or cable slings and large metal pulleys. In order to maintain the wire cable capacity, the ratio between the diameter and type of cable and the diameter of the pulley is referred to as a D/d ratio where “D” is the diameter of the pulley and the “d” is the diameter of the cable. By international standards for existing technology, this ratio ranges from 12-15. For example, a one inch diameter cable would require a pulley having a minimum diameter of twelve inches. Wire cables and their associated pulleys are commonly made of steel or some other metal which results in the cables and pulleys being extremely heavy and cumbersome. One form of cable and pulley rigging system is produced by Meadow Burke and illustrated on their website of MeadowBurke.com.
Multi-strand synthetic fiber slings are commonly manufactured in two styles. In an eye to eye style, the sling is made with a continuous fiber core, and at each end the fibers are equally split, placed in a protective cover and sewn together. A continuous loop style is made similar, but the fibers are separated for the entire loop, creating a continuous endless loop, and the fibers are placed within a protective sleeve. Both of these styles are usually made in two different configurations, either single path or twin path. The single path has one or several continuous fibers looping around for a predetermined number of times depending on the required strength. The twin path has two separate paths of one or more continuous loops in isolated paths, and a protective cover is sewn lengthwise around the twin paths.
Flat type synthetic slings are manufactured in many styles. They are a woven fabric type materials that can be sewn together in plies to increase strength and are supplied in variable widths. They can be eye to eye with either sewn or attached metal ends, or a reverse eye which changes orientation of the eye in relationship to the body. They also can be sewn together to create a continuous loop type sling. Such slings are generally used for lighter loads. Examples of synthetic fiber slings are disclosed in U.S. Pat. Nos. 4,850,629, 5,492,383, 5,836,631, 6,508,051, 7,658,423, 7,661,737 and 7,669,904.